This invention relates to a photomask used as a photolithography mask for fine working in the process of manufacturing a semiconductor integrated circuit device or the like, to a photomask blank that is used to make this photomask, and to methods of manufacturing the same.
Photolithography has come to be used in recent years in the process of forming wiring and other regions in the manufacture of semiconductor integrated circuit devices. Known photomask blanks used as the exposure original in the photolithography process include those with a basic structure in which a shading film (light-shielding film, opaque film, non-transmitting film) of chromium (Cr) is formed over a transparent substrate, and those with a multilayer structure in which an anti-reflective film such as a chromium oxynitride (CrON) film is further laminated in order to prevent reflection on the shading film (light-shielding film, opaque film, non-transmitting film) surface produced by the exposure light.
The method employed to manufacture a photomask blank such as this involves introducing a transparent substrate into a vacuum chamber in which a sputtering target has been disposed, and forming a shading film (light-shielding film, opaque film, non-transmitting film) over the transparent substrate by reactive sputtering. With this film formation method, it is possible to raise the sputtering power in order to enhance the photomask blank productivity. Unfortunately, while raising the sputtering power does raise the film formation rate (that is, the deposition rate), if any impurities are present in the target, there is the possibility that there will be a higher incidence of particle generation in the formed thin film, which lowers the yield.
In view of this, in an effort to enhance productivity through a higher yield, the inventors investigated lowering the sputtering power in order to lower the film formation rate (that is, the deposition rate). It was found, however, that merely focusing on lowering the power (sputtering power) as one of the sputtering conditions only results in the following new problems.
Specifically, it was learned that if the film formation rates of the thin films that make up the photomask blank are each lowered, in general, the crystal grains of the film deposited on the transparent substrate will be larger, and this will be attended by extremely large film stress caused by the crystal grains pulling on one another. The mechanism by which this film stress is generated is not entirely clear, but it is thought to be attributable to the deposition rate of the films. In the case of a photomask blank made up of chromium-based thin films, such as a photomask blank having a three-layer structure of CrN/CrC/CrON, it was found that this problem is particularly serious with the CrC thin film, which is the thickest of the three. Further examination of the situation by the inventors revealed that film stress also greatly affects the film materials that make up the thin films. This problem with stress was found to be particularly serious with chromium-based thin films, with chromium carbide films in which the chromium contains carbon, and with chromium oxide films in which the chromium contains oxygen.
Tensile stress occurs and leads to substrate warping in a photomask blank including thin films manufactured through a series of manufacturing steps and having the above-mentioned film stress, and in the photomask obtained by patterning this photomask blank. Therefore, if a photomask is produced from a photomask blank such as this, there is the danger that the patterning precision will not be as designed, producing defective products. Specifically, wiring design is important in the manufacture of a semiconductor integrated circuit device, and if a photomask such as this is used to transfer a pattern to a semiconductor wafer or the like, the pattern will not be formed as designed on the semiconductor wafer, resulting in circuit malfunction, and therefore this photomask cannot be used and is a defective product.
In view of this, the inventors investigated a method of manufacturing a photomask blank with which it is possible to avoid the problem of film stress in thin films formed under film formation conditions (film sputtering conditions) involving sputtering at low power as mentioned above.
Aside from the sputtering power conditions, the material of the thin films that make up the photomask blank is determined on the basis of the following sputtering conditions, and the inventors therefore conducted investigations and experiments into various parameters.
First, they investigated the gas pressure used in sputtering. Gas pressure was the only sputtering condition they focused on, and other parameters were kept constant in the experiments and investigations.
FIG. 11 is a graph of the relationship between the gas pressure during the formation of CrC and the change in substrate warping attributable to film stress for a photomask blank in which CrN/CrC/CrON is formed over a transparent substrate. Here, the change in substrate warping is the change in flatness, and this change in flatness is defined as the difference between the initial flatness (the flatness of the transparent substrate at the outset) and the flatness after the thin films have been formed over the transparent substrate. As to the symbols used for the change in flatness, a minus sign indicates a change in tensile stress, and a plus sign indicates a change in compressive stress. Flatness was measured using an AS8010 made by Tropel. It is clear from this graph that there is less change when the gas pressure is lower, and more change when the gas pressure is higher (the film stress is greater). In other words, these investigative results reveal that it is undesirable for the gas pressure to be raised because the deposition rate will be too low and the change in flatness will be greater (film stress will be greater), and that it is undesirable for the gas pressure to be lowered because film formation stability (film sputtering stability) will be poor.
Next, the types of gas that make up the mixed gas were investigated. Experiments and investigations were conducted for when the gas components were the only sputtering condition examined, the power and gas pressure were not varied, and other parameters were set to constant values.
A method in which a reactive gas is mixed into an inert gas for the gas used in sputtering is generally employed to control the stress of the films. The inventors first turned their attention to the reactive gas, and conducted sputtering using nitrogen (N) as the reactive gas, for example. As a result, it was found that if the amount of nitrogen is too large, abnormal discharge will generate particles. It was thus revealed that there is a limit to how much reactive gas N (nitrogen) can be introduced, so the components of the mixed gas must be used in appropriate amounts (the amount of reactive gas in the inert gas). For instance, when a photomask blank comprising an anti-reflective film of CrON formed over a shading film (light-shielding film, opaque film, non-transmitting film) of CrC is used for a low-wavelength application (such as 365 nm), the CrON must be made thin due to optical characteristic requirements. The amount of nitrogen monoxide (NO) gas introduced as the reactive gas was reduced in order to make the CrON film thinner, but this generated film stress due to a decrease in nitrogen (N) atoms in the CrON film. On the other hand, excessively increasing the amount of nitrogen monoxide (NO) gas introduced as the reactive gas during the formation of the above-mentioned CrON film resulted in abnormal discharge during sputtering due to the effect of the nitrogen (N) atoms, and consequently the above-mentioned problem with particles resulted in poor film quality. Therefore, it was revealed that even if the amount of nitrogen (N) introduced as the reactive gas is optimized, it will still be very difficult to achieve optimal sputtering conditions in terms of the relation between introduction amount and film quality.
Thus, when films were formed by changing the various parameters of the film formation conditions (film sputtering conditions) for film stress, namely, the gas pressure, gas flow ratio, sputtering power, and so forth, it was extremely difficult to control both the optical characteristics and the film quality (changes in the optical characteristics of the formed film, changes in film quality, etc.). Therefore, an attempt was made to achieve the above-mentioned goal by looking for an inert gas with good controllability as the type of gas that makes up the above-mentioned mixed gas under the condition that the power during sputtering will be lowered.
The present invention was conceived in light of the above problems, and it is an object thereof to provide a photomask blank and photomask that have thin films with low film stress, have good film quality, and can be mass-produced at a high yield, which is accomplished by changing the constitution of the mixed gas, and to provide methods of manufacturing the same.